The present invention relates to low cost catalytic articles and methods for treating a fluid stream, e.g., a gaseous fluid stream. Among other things, the articles and methods disclosed herein are well suited for converting pollutant components in exhaust streams produced by small engines to innocuous components. The exhaust gases of internal combustion engines, including small engines, are known to contain pollutants such as hydrocarbons, carbon monoxide and nitrogen oxides (NOx) that foul the air.
More stringent emission regulations for devices powered by small internal combustion engines are increasingly being mandated by various regulatory agencies. By small engines, it is meant that the engines, usually two-stroke and four-stroke spark ignition engines, have a displacement of less than about 75 and preferably less than 35 cubic centimeters. Such engines (“utility engines”) are found, in particular, in gasoline-engine powered lawn mowers, motorized chain saws, portable generator units, snow blowers, grass/leaf blowers, string mowers, lawn edgers, garden tractors, motor scooters, motorcycles, mopeds, and like devices. Such engines provide a severe environment for a catalytic exhaust treatment apparatus. This is because in small engines, the exhaust gas contains a high concentration of unburned fuel and unconsumed oxygen. Since the users of many of such devices (e.g., motorized saws, lawn mowers, string cutters) work in close proximity to the devices, the concern for reducing the emissions is heightened.
Exhaust treating catalyst articles offer one solution toward reducing emissions from devices powered by small engines. However, practical integration of catalytic articles into such devices can be difficult because the operating conditions for small engines pose difficult design challenges.
First, the catalyst article must be durable. In comparison to devices powered by larger engines (e.g., an automobile), devices powered by smaller engines are less able to absorb and diffuse the vibrations caused by the engine. The vibrational force in a two-stroke engine can be three or four times that of a four-stroke engine. For example, vibrational accelerations of 70 G to 90 G (G=gravitational acceleration) at 150 hertz (Hz) have been reported for small engines. The harsh vibration and exhaust gas temperature conditions associated with small engines lead to several modes of failure in the exhaust gas catalytic treatment apparatus, including failure of the mounting structure by which a catalyst member is secured in the apparatus and consequential damage or destruction of the catalyst member due to the mechanical vibration and to flow fluctuation of the exhaust gas under high temperature conditions. In addition, small engines provide less design flexibility with regard to the placement of the catalytic article. In devices powered by small engines, the close proximity of the catalytic article to the engine exposes the article to intense vibrations. Furthermore, small engines are characterized by high temperature variations as the load on the engine increases and decreases. Accordingly, a catalyst member used to treat the exhaust of a small engine is typically subjected to greater thermal variation and more vibration than the catalytic converter on an automobile, and these conditions have lead to spalling of catalytic material.
Second, the catalytic articles preferably accommodate high flow rates since the majority of small engine platforms exhibit high space velocities due to the limited size of the mufflers employed on these engines. For instance, a small engine having a displacement of 50 cubic centimeters operating with a maximum of 8,000 rpm typically has an exhaust output of 12,000-15,000 L/h. Catalyst articles that significantly restrict the flow rate of the exhaust stream are less desirable since higher backpressures within the exhaust system reduce the engine's operating efficiency. Moreover, as a result of the high flow rate of exhaust stream through the catalyst article, the catalyst composition employed must be highly active and optimally disposed within the article to ensure adequate pollutant conversions.
Third, the catalyst articles are preferably lightweight and occupy small volumes since many of the devices powered by small engines are handheld tools, e.g., weed trimmers, chainsaws. Excessive weight or unwieldy protrusions from such devices negatively restrict the applications that the devices were designed for.
Fourth, the cost of the emissions treatment system cannot significantly increase the overall cost of the device to ensure that the device remains competitive on the marketplace. Small engines typically power moderately priced devices. Accordingly, a need has arisen to design a catalytic article for treating the emissions of devices powered by small engines which meets expected standards, yet minimizes the added cost to the device.
Catalysts useful in small engine applications are described in U.S. Ser. No. 08/682,247, hereby incorporated by reference. Briefly such catalysts comprise one or more platinum group metal compounds or complexes, which can be on a suitable support material. Suitable support materials include refractory oxides such as alumina, silica, titania, silica-alumina, aluminosilicates, aluminum-zirconium oxide, aluminum-chromium oxide, etc. The catalytic materials are typically used in particulate form with particles in the micron-sized range, e.g., 10 to 20 microns in diameter, so that they can be formed into a slurry and applied as a washcoat on a carrier member. Suitable carrier members may be employed, such as a honeycomb-type carrier of the type having a plurality of fine, parallel gas-flow passages extending therethrough from an inlet or an outlet face of the carrier so that the passages are open to fluid-flow therethrough. The coater carrier is disposed in a canister suited to protect the catalyst member and to facilitate establishment of a gas flow path through the catalyst member, as is known in the art.
Emissions requirements have become increasingly stringent, requiring development of both new catalysts and higher catalyst loadings. In addition to absolute emissions standards, emissions control system longevity, i.e. “durability”, requirements have also been extended. This maintenance of operation requirement over extended periods has also challenged catalyst development, and has required still further increased catalyst levels. It is primarily the catalyst loading levels, in particular, precious metal loading, which controls the cost of the catalytic converter. Converters that meet all the requirements with a minimum of precious metal loading is a primary objective of catalyst manufacturers.
Numerous reactions can occur during combustion of a hydrocarbon fuel in a variety of temperature and fuel/air stoichiometric environments and the products of such reactions can limit catalyst durability. For example, it was recognized quite early that lead, formerly supplied as an octane booster in fuel as tetraethyl lead, was a serious catalyst poison. The lead octane boosters, thus, have been removed from modern day fuels. However, numerous trace elements still come into contact with the automotive exhaust catalysts, some unavoidably so, and several of these are known to decrease catalyst durability. Not all these poisonous trace elements are derived from the fuel.
For example, zinc dialkyldithiophosphates (ZDDPs) have been long used as antioxidants and/or high-pressure lubricant additives in motor oils. Especially with modern high-speed engines, increased piston/wall clearances and decreased sealing allow increased entry of oil into the combustion chamber, where oil additives, or their combustion byproducts, subsequently pass into the exhaust stream. Such catalytic poisoning is one of the primary obstacles to the durability of low emission catalyst systems. Trace amounts of zinc, phosphorus, calcium, and other elements are put in engine oil as anti-wear additives. The purpose of such additives is to protect engine parts from excessive wear during start-up, when engine oil is not coating the metal components of the engine. However, as the engine burns oil, zinc and phosphorus are exhausted through the catalytic converter, which may accelerate degradation of exhaust catalyst activity. Although the antiwear additives could be removed from the oil, long-term durability of the engine could suffer.
The use of engine anti-wear additives, such as phosphorous and zinc, is described in many references. These additives include compounds such as ZDDPs, also referred to as zinc dithiophosphates (ZDTPs), and zinc dithiocarbamates (ZDTCs). Other disclosed zinc and phosphorous additives to oil include metallic detergents included as extreme pressure agents. Reference is made to U.S. Pat. Nos. 4,674,447 and 5,696,065. The phosphorous and zinc are disclosed as lowering the function of the motor vehicle exhaust treatment catalyst.
Oil additives, such as ZDDP, form an antiwear coating on engine components and act as an antioxidant in the oil. Although engines are designed to minimize the quantity of engine oil exiting the engine via the combustion chamber and exhaust system, it is inevitable that a small fraction of engine oil is released by this mechanism. The ZDDP additive of engine oil deleteriously affects catalytic converters due to phosphorus from the ZDDP interfering with active sites within the catalyst. These phosphorus containing species deposit onto, or react with washcoat components, such as aluminum oxide and cerium oxide, and remain there indefinitely. This phenomenon is commonly referred to as “phosphorus poisoning.”
This poisoning mechanism is quite complex, and highly dependent upon the operating temperature, the oil consumption of the engine, and the source of the oil consumption. For example, when oil leaks past the piston rings, and washes into the combustion chamber, the oil goes through the combustion process. This will result in certain types of phosphorus and/or zinc compounds (among other contaminants). Particular compounds may have a particular deactivation effect on the catalytic converter, depending upon the operating condition. On the other hand, oil that leaks past the exhaust valve guide and stem, may not go through the combustion process, and result in a different type of poisoning of the catalytic converter.
Measures to eliminate or reduce ZDDP in engine oils have been investigated. Alternatives to ZDDP have been produced which have been shown to provide antioxidant and antiwear properties similar to ZDDP. However, the ZDDP alternatives are cost prohibitive. Engine oils may be formulated with a lesser amount of ZDDP with the consequences that engine wear and oil oxidation increase, the former limiting engine life and the latter reducing useful oil life.
Another method of removing poisoning inorganic species is through the use of a trap. For example, an upstream foam trap could be used to trap inorganic species from poisoning downstream catalytic converters. U.S. Pat. No. 6,810,660 discloses the use of an exhaust aftertreatment system for an internal combustion engine, which mitigates deleterious poisoning of a catalytic converter by a phosphorous containing species. The system comprises a trap in the exhaust duct located upstream of the catalytic converter. In one embodiment disclosed in the '660 patent, the trap may be a ceramic or metallic foam trap. However, while the '660 patent may suggest in general, the use of a foam trap, the patent does not discuss any specific requirements for the use of such foam traps in small engines.
Foams have been used as catalyst supports. For example, U.S. Pat. No. 4,774,217 discloses a three-dimensional catalytic structure for cleaning exhaust gas from automobiles. According to the '217 patent, the catalytic structure is produced by contacting a ceramic carrier with a catalyst-containing solution and drying the carrier by evaporating the solvent in the solution so that the catalyst is carried at a higher density in the activated alumina layer. The foamed carrier of the '217 patent is made using a polyurethane foam and applying a ceramic material to the surface of the polyurethane foam. The ceramic material is subsequently calcined and coated with a catalyst slurry. However, the ceramic foam of the '217 patent would not survive small engine stresses.
Small engines and motorcycles often require engine oil mixtures that are high in inorganic components for proper engine lubrication. These oils whether applied directly to the fuel or engine components, often make their way to the catalyst. Oil species that include: P, Zn, Ca, Si, Cr, Pb and others are well documented poisons to the catalyst. Furthermore, the catalyst volumes for these systems are small because of muffler design and varying emission requirements and as a result, tend to poison much more rapidly than other applications, such as automotive. Therefore, it is an objective of the present invention to provide a means of removing inorganic compounds from exhaust, thereby preventing catalytic poisoning.